The present invention relates to location and movement of remote operated vehicles of the type for use in exploration of an underwater environment.
Remote operated vehicles (ROVs) of this type typically comprise an ROV system having a remote operated “submarine” unit or vehicle (a fish), a land- or ship-based remote control unit (a topside) and an umbilical cable for connecting the fish and the topside together and carrying signals between the fish and the topside. The fish may be powered by an onboard power unit or by a power unit located in the topside, in which case power is conveyed to the fish via the umbilical cable. The ROV system can be for commercial or leisure purposes.
The fish is usually fitted with swimming or propulsion means, such as motor driven propellers or thrusters, which are used for maneuvering the fish underwater. Also, the fish typically carries one or more video or stills cameras. Images from the cameras can be transmitted from the fish, via the umbilical cable, to the topside for displaying on a monitor or viewfinder attached to the topside. The images can be used both for observational purposes, and as visual input for a user to steer the fish along a desired path by use of controls provided on the topside. Additional devices for measuring parameters such as speed and depth can also be provided on the fish. Readings from these are also sent to the topside along the umbilical cable.
The topside is used to transmit control signals down the umbilical cable to the fish for controlling both the thrusters, and any features such as grabbers (movable arms having gripping means for picking up articles from the seabed), pan and tilt mechanisms fitted to the cameras, and lights.
ROV systems are typically costly pieces of equipment, so it is important to ensure that the fish, once deployed on an exploration trip, can be navigated accurately to avoid the risk of collision, and also safely returned to the topside after use, or recovered in some other way. This is of particular significance in situations where it is not possible for the user to drive the fish by using controls to steer the fish in response to images from the camera. This may occur if, for example, the camera is damaged or the umbilical cable is severed or damaged.
One potential way of achieving this is to provide the fish with an inertial navigation system (INS). This is an arrangement of inertial sensors in the form of gyroscopes (e.g. fiber-optic gyros) and accelerometers which continuously monitor the motion of the fish, and a processor to process the output of these devices. By successively measuring the time spent moving in a given direction at a given speed, it is possible to calculate the path travelled by the fish, and hence its position relative to its starting position (typically its launch position from the topside). Hence it can be directed to a desired location. At the end of the trip, the relative position of the fish is known. The processor can then take control of the thrusters, and drive the fish back to the topside without input from the user.
INSs are commonly used on aircraft, spacecraft and missiles to aid navigation and accurately determine position. Typically, good results are achieved. However, the systems are less well suited to the guidance of ROV system fish. The strong currents and turbulence experienced by a fish when underwater tend to contribute a large error to the INS results, making any measurements insufficiently accurate to be of use. This is particularly the case with recreational ROV systems, in which the fish is preferably small and light and hence more prone to being buffeted by underwater forces, and may also be driven in a unskilled manner. Also, the results achieved using INS depend on comparing the present calculated position with a start position. This means that the INS will only be able to correctly return a fish to its topside in situations where the topside has not moved since launch of the fish, which is not always the case. For example, the topside may be deployed from a moving boat.
An alternative approach is to use ultrasound signals to determine the position of the fish. An ultrasound signal can be emitted from the topside through water to a receiver on the fish; the time taken indicates the distance of the fish from the topside because the speed of sound in water is constant. However, while this gives a satisfactorily accurate measure of distance, it is not well-suited to determining the location of the fish. It is necessary to have a initial idea of the fish's approximate whereabouts to ensure that the ultrasound is broadcast in the correct direction. This can be improved by using further ultrasound transmitters, for example, positioned on buoys. However, this complicates the ROV system, and makes it less transportable and hence less desirable for leisure applications. Also, more complex calculations need to be performed to determine the position.
Thus it is desirable to provide an ROV system having a fish which can be accurately navigated and located.